Electro-photographic apparatus

ABSTRACT

In the development of electrostatic latent images with liquid electrophoretic developer utilizing a development electrode, the spacing between the electrode and the latent image is made selectively adjustable, to control the degree of background fog occassioned by the electrode effect and the extent of edge phenomenon present in the print.

United States Patent 1191 Fukushima et a1. Sept. 3, 1974 [54] ELECTRO-PHOTOGRAPHIC APPARATUS 3,577,259 5/1971 Sato et al. 117 37 1 Fukushima; Masamichi saw; 3:235:23? 18/1335 1232215211711: 3111/3 3; SeiJi Matsumotm all of Asaka, Japan 3,707,139 12 1972 Fukushima et al. 118/637 [73] .ASSignee: Rank Xer Ltd. London" 3,744,452 7/1973 Sav1t et a1 1l8/DIG. 23 g y Said Fukushima and 3,753,419 8/1973 Fukushlma et al. ll8/D1G. 23 Matsumoto FOREIGN PATENTS OR APPLICATIONS 22 Filed: Man 13 1972 948,138 1/1964 Great Britain 118/637 X [21] Appl 234,285 Primary ExaminerRobert R. Mackey Assistant ExaminerLeo Millstein 52 US. Cl 118/637, 117/37 LE, l18/DIG. 23 Attorney, Agent, or Firm-James J Ralabate [51] Int. Cl G03g 13/00 [58] Field of Search 118/637, DIG. 23, 429; [57] ABSTRACT 117/37 A; 355/10 In the development of electrostatic latent images with liquid electrophoretic developer utilizing a develop- [5 6] References C'ted ment electrode, the spacing between the electrode and UNITED STATES PATENTS the latent image is made selectively adjustable, to con- 3,l47,147 9/1964 Carlson 118/637 trol the degree of background fog occassioned by the 3,299,787 1/1967 Kolb et al. Il8/DIG. 23 electrode effect and the extent of edge phenomenon 3,328,193 6/1967 Ollpl'lfll'lt 61; a1. 117/37 present in the print 3,415,223 12/1968 Zweig 118/637 0 3,556,050 1/1971 Trachtenberg 118/637 4 Claims, 12 Drawing Flgures 24 g 24 25 15 5 29 26 /9 f l5 .1 1 26 111-1111111111 q" I 9 1' 6 6 o 2 3 II UL l Alli] 2 2 PATENT ED SEP 31924 sum 10F 4 zzzz: f

PATENTEDSEP 31914 sum anr 4 ll; llllllllll 3 lllll 1 ELECTRO-PI-IOTOGRAPI-HC APPARATUS INTRODUCTION AND SUMMARY OF INVENTION .oxide particles embedded in an insulating resin binder,

and this layer may be formed as a coating on a backing sheet of relatively conductive paper. In use, the photoconductive layer is first uniformly electrostatically charged over its surface and then subjected to an optical image exposure. This results in an imagewise discharge of the surface charge in accordance with the optical pattern to obtain a latent electrostatic charge image on the surface of said photosensitive layer. This latent image can then be developed by immersing the image surface in a liquid developer, comprising a suspension of colored toner particles in an electrically insulating liquid vehicle, whereby the toner is attracted to and deposited on the charged areas in accordance with the charge density.

It is known that in this electrophotographic process, toner deposition tends to take place preferentially in the areas where the charge potential changes from a lower to a higher value, and to a lesser extent in the areas where the electrostatic charge is uniformly distributed, resulting in what is commonly referred to as edge effect. In order to prevent or minimize this phenomenon, development is often carried out in the presence of a developing electrode positioned in close proximity to the photosensitive surface carrying the electrostatic charge image thereon. However, this edge effect is not necessarily undesirable and can be advantageously utilized in certain situations, such as with two tone line images of alpha-numeric text material; while in other situations the effect is decidedly undesirable, as for example when seeking to reproduce continuous tone photographic images.

It is therefore an objective of the present invention to control the edge effect phenomenon, utilizing it when helpful in enhancing the image reproduction, and eliminating it when its effects are undesirable. Since the edge effect can be essentially eliminated by locating a development electrode in close proximity to the electrostatic latent image surface, but is quite pronounced in the absence of a development electrode, the present invention provides an adjustably movable development electrode structure. Depending upon the nature of the image being reproduced, the positioning of the electrode relative to the image surface being developed is adjusted so as to obtain maximum edge effect, minimum or substantially no edge effect, or a value of edge effect between these two extremes.

It is therefore one object of the present invention to provide for selective control of the edge effect phenomenon in the development of an electrostatic latent image with a liquid electrophoretic developer.

Another object of the present invention is to provide such edge effect control by selective adjustment of the spacing between a development electrode and the latent electrostatic image surface.

Other objects and advantages of the present invention will become apparent to those skilled in the art from a consideration of the following detailed description of several illustrative embodiments of the invention, had in conjunction with accompanying drawings, in which like numerals refer to like or corresponding parts, and wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a distribution ofelectrostatic latent image charge on the surface of an electrophotographic element surface;

FIG. 2 graphically illustrates the distribution of electric field corresponding to the charge distribution shown in FIG. 1 as effective for inducing the deposition of toner;

FIG. 3 illustrates a different distribution of electrostatic latent image charge on a photosensitive surface;

FIG. 4 illustrates the distribution of electric field corresponding to the charge distribution shown in FIG. 3;

FIG. 9 illustrates another embodiment of developing apparatus utilizing the principles of the present invention;

FIG. 10 is an isolated face view of a development electrode used in the apparatus of FIG. 9; and

FIGS. 11 and 12 illustrate two additional embodiments of the present invention.

DETAILED DESCRIPTION FIG. 1 depicts a photosensitive sheet 1 with a latent electrostatic charge image represented on its image surface. The letter A indicates a relatively large area with a substantially uniform charge 2 distributed thereover, and corresponding for example to a large black area in the image. The charge, which is indicated as negative in this drawing, can of course be positive, if desired. B indicates a relatively small and narrow area provided with electrostatic charge 3, corresponding for example to a black line in the image, such for example as a portion of a letter.

The electric field generated by this electrostatic charge pattern and effective for attracting developer toner, takes the form indicated in FIG. 2. The solid line portion 4 indicates the electric field corresponding to the area A in FIG. 1. The line 13 indicates zero level, the areas above and below line 13 indicating toner attractive and replusive forces respectively. As shown in this figure, area A generates an electric field whose effect is most strongly attractive of developer toner at the edge portions thereof, with only negligible toner attractive forces in its central portion. As a result, during development toner would depositpreferentially at said edge portions, leaving the central portion substantially free of deposited toner. This phenomenon is extremely undesirable when seeking to reproduce a continuous tone image. On the other hand, the electric field corresponding to the narrow area B and depicted by line portion 5 shows no breakdown in the central portion because of its narrow width, thus being capable of a completely developed image.

In order to prevent this edge effect, namely toner deposition concentrated at the edge or peripheral portions of a large dark image area, a developing electrode is usually provided in close proximity to the image surface of the photosensitive sheet carrying the electrostatic charge image. The dotted line 6 in FIG. 2 indicates the electric field pattern corresponding to the area A of FIG. 1 when a developing electrode is provided. In this case the electric field is nearly proportional to the distribution of the electrostatic charge without any edge effect, and therefore is suitable for the reproduction of continuous tone images. On the other hand, in the area B, as shown by dotted line 7, the electric field does not show any significant change resulting from the development electrode, except for some general increase in potential.

Thus, it is apparent that a development electrode is quite advantageous for reproducing continuous tone images. On the other hand, it has been experimentally confirmed that a strong edge effect is advantageous in certain other instances, as for example when the origi nal image is principally composed of alphanumeric characters. For example, if an original image of test material has a uniform background coloration, a retentive background charge 8 will appear over the entire photosensitive layer 1, as indicated in FIG. 3. The part C has a larger amount of electrostatic charge, which may correspond to the black line of a letter present on the colored background of the original image. Also, apart from the presence of background coloration, certain types of photoconductive sheets show a certain amount of uniform residual charge over their whole surface, it not being practically possible to discharge the entire charge in portions that are struck by light during the image exposure step. In these cases, during development, if a development electrode is used with high effectiveness at a sufficiently close distance to the latent image surface, the entire electric field attracts the toner proportionately to the distribution of electrostatic charge, as indicated by the dotted line 11 in FIG. 4. This causes a uniform background deposition of toner, and consequently a very high background fog level. However, the electric field indicated by the solid line 10, obtained in the absence of a development electrode, does not have a toner attracting background electric field. Consequently, development under this condition provides clear images of the text material with very low background fog. The broken line 12 indicates the electric field when the distance between the developing electrode and the latent image is increased above that providing the field depicted by line 10. The larger the spacing between the development electrode and the image surface, the less is the effect of the electrode on the electric field. Utilizing this variable, one can seek optimum compromises between image density. background fog, and image resolution.

In electrophotographic reproduction of line images such as alpha-numeric text material, it is frequently observed that the resolution is considerably deteriorated by the deposition of toner along, but outside of the actual image line areas. This effect is often occassioned by mechanical disturbances of the deposited toner during development. When a development electrode is located a significant distance from the electrostatic image, or is not used at all, the electric field at the image edges extends below the base or zero line 13, as indicated by line 10 or line 12 in FIG. 4. Thus, the electric field at the edge of image area C is repulsive of the toner, and is therefore capable of preventing the deposition of toner outside the actual image area, and thereby contributes to sharp developed images.

Thus, from the foregoing description, it is generally found preferable that a development electrode be used in close proximity to the electrostatic image when reproducing continuous tone images, and be used at a greater distance from the image for reproducing two tone line images. The following embodiments of the invention are directed toward providing for adjustable development electrode spacing.

In the electrophotographic developing apparatus of FIG. 5, a photosensitive sheet ll having a latent electrostatic charge image on its upper surface is passed between support rolls 21 and the development electrode rolls 15. The sheet is advanced from left to right in the drawing by rotation of the support rolls 21 in the direction indicated by the arrows 23. The latent image is developed during this traverse by means, for example, of liquid developer 25 (consisting of toner particles dis persed in an insulating liquid) supplied by the nozzles 24. This developer flows over sheet 1 into tank 60, and is recirculated from drain 61. The support rolls 2] can be made either of electroconductive or insulative material. The development electrode rolls, however, are made of electroconductive material, and their cylindrical surfaces are shown as being in very close proximity to the upwardly facing image surface of sheet 1, thereby realizing development without edge effect. The indicated development roll arrangement is therefore appropriate for reproduction of continuous tone images.

It is desirable to use four to eight development rolls since only one roll is normally unable to provide sufficient development. The diameter of the rolls is preferred not to exceed about 40 mm. in order to obtain a compact apparatus. The advancing speed of the photosensitive element is preferred to be around 10 mm./sec., since lower speeds would result in low efficiency for the apparatus, while higher speeds would result in insufficient developing time and the formation of streaks on the developed image due to mechanical disturbance of deposited toner. The axles 17 for the development rolls 15 are rotatably mounted in holes 18 fonned in beam 29. The holes 18 are slightly elongate, allowing a small amount of vertical displacement of the rolls to accommodate differences in thickness between photosensitive elements 1.

As previously stated, the condition shown for the apparatus of FIG. 5 is one to obtain maximum effect from the development electrode, resulting in no, or very little edge effect. However, the edge effect may be preferred when reproducing line images, and therefore a mechanism is provided which is capable of vertically lifting the development rolls, and thereby decreasing the development electrode effect. As shown in FIG. 5, the beam 29 which carries the development electrode rolls 15 is supported at its ends by bearings 19 and 19 on support beams 20 and 20'. Beams 20 and 20 can be moved from the position shown in the directions indicated by the arrows 22, causing bearings 19 and 19 to ride up on camming steps 26, thereby elevating the whole beam 29 together with the development rolls 15. In this state the development rolls are not as close to the image surface of the photosensitive element 1, resulting in development with an increased edge effect. This arrangement of the apparatus is therefore appropriate for obtaining reproductions from line images with low background fog level.

The apparatus of this invention thus provides a mechanism for adjusting the spacing between an image sheet 1 and the development electrode rolls 15, to promote development without edge effect when reproducing continuous tone images, and to promote development with significant edge effect when reproducing two tone images consisting principally of lines or alpha-numeric characters.

The apparatus shown in FIG. 6 is a modification of the apparatus in FIG. 5, particularly of the development electrode roll support and adjustment structure. In FIG. 6, the bar 29 supporting the electrode rolls is pivoted at to a fixed support, while its other end is adjustable in height by the support bearing 19 and movable stepped camming beam 20. This structure is used as shown when reproducing continuous tone images, but the free end of bar 29 is elevated by moving the support beam 20 in the direction of the arrow when reproducing line images.

FIG. 7 shows another variation in which the first two rolls 15 are mounted on a fixed bar 29a, while the second two rolls 15 are mounted on bar 29b which is movable in exactly the same way as bar 29 in FIG. 5.

The modifications of FIGS. 6 and 7 are useful in instances where it is undesirable to have development electrodes placed too close to the surface of the photosensitive sheet 1 in the initial stages of the development process even when the reproduction is of a continuous tone image. With all the electrodes positioned for maximum effect, there is a tendency for fogging due to full development of the slight residual or background potential in the background areas. The present embodiments minimize the development of this background fog.

FIG. 8 is a detailed view of a developing roll 15. The roll is formed of electroconductive material, and is provided with slightly enlarged diameter portions 15 and 15" at both ends. The central portion 30 with the slightly smaller diameter functions as the development electrode, while the end portions 15' and 15" contact the non-image side edge portions of the photosensitive sheet 1 in lowered position. In this condition, the distance between the image surface of the photosensitive element and the central portion 30 of the roll is determined by the difference in diameter between the portions 15, 15" and the portion 30. This difference is preferred-to be about 0.0050.5mm. It is also possible for these rolls 15 to have a uniform diameter over their whole lengths. This is acceptable when using a photosensitive layer for sheet 1 which consists of a coating containing photoconductive powdered material dispersed in a resinous binder, because the image surface contacted by rolls 15 contains minute irregularities. Consequently, the electrostatic charge image is not destroyed even if the rolls do contact the image surface directly. so long as said irregularities exceed necessary limits. 2

FIGS. 9 and I0 illustrate another fonn of developing apparatus utilizing developing rolls 31combined with a different structure for controlling the spacing between the rolls and the image surface. The rolls 31 have their axles 34 mounted in holes 35 provided in a fixed immovable beam 33. The holes 35 are elongate to allow vertical movement of rolls 31 over a fairly wide range. Two flexible belts 32 engage the rolls 31 adjacent their ends, in either a first set of annular grooves 40, or a second set 41. Guide means 36 is provided for holding and shifting the belts 32 between the two sets of grooves.

In operation, support rolls 21 are rotationally driven in the direction indicated by arrows 23 about fixed axes, and thus transport the photosensitive sheet 1 from left to right in the drawing. During this advancement the side edge portions of said photosensitive sheet are engaged by said belts 32 provided on the development rolls 31, under the weight of these rolls. The electrostatic charge image on the photosensitive sheet 1 is developed during its passage under the development rolls 31 by liquid developer supplied from above by nozzles 24.

As shown in FIG. 10, annular grooves 41 are not as deep as annular grooves 40, and belts 32 are located in one or the other set of these recesses. The thickness of V the belt is a little larger than the depth of the deeper recesses 40, and therefore the outer surfaces of the belts protrude slightly from these grooves to the level of broken line 37. In this condition the image surface of photosensitive sheet 1 is located at this broken line position, which is preferably a distance of about 0.050.5 mm. from the surface of rolls 31, thereby causing said rolls to act effectively as development electrodes to obtain development without edge effects. On the other hand, when the belts 32 are shifted to the shallow recesses 41, as indicated in dotted lines in FIG. 10, the outer surfaces of the belts protrude a greater distance from the roll surfaces, placing a greater distance between the image surface of sheet 1 and the roll surfaces, as indicated by the broken line 38, thereby reducing the effect of the rolls 31 as development electrodes and obtaining reproduction with decreased background fog and increased edge effect.

This shifting of the belts is carried out by displacing the guide 36, as will be fully understood by those skilled in the art. Since the belts 32 are longer when engaged in grooves 41 than when in grooves 40, they may be made of a resiliently stretchable material, or may include a small section of such material.

The embodiment shown in FIG. 11 is a variation of those in FIGS. 57, and the same reference characters refer to components having the same structure and function, and need not be repeated here. In this embodiment, a'different mechanism is provided to displace the development rolls 15 vertically to regulate the development electrode effect. Rolls 15 are supported on a beam 45, which is pivotable about pin 46 by shifting the handle portion 48 in the directions of arrow 47. This pivotal action elevates or lowers the rolls 15 away from or towards the backing and support rolls 21, thereby providing for control of the edge effect and background fog, as previously explained. The pivotal action obviously provides a differential spacing and consequent differential effect for the electrode rolls 15, for the purposes and advantages previously described in connection with FIGS. 6 and 7.

The foregoing embodiments of the invention all use rollers as the development electrodes. Because roll type development electrodes can be safely located very close to the surface of a photosensitive image carrier, they are extremely effective for suppressing or controlling the edge effect. It is however quite possible to apply the techniques of the present invention to other forms of developing electrodes, and this is illustrated in FIG. 12.

In FIG. 12 the photosensitive element l is advanced by feed rolls 50 and 50' into liquid developer bath 52, where its latent image is developed during its advancement along the curved guide plate 55. A corresponding curved plate development electrode 54 is positioned in close proximity to the upwardly facing image surface of sheet 1 in its movement through the developer. The liquid developer 52 is supplied from a nozzle 53 to the upper surface of said developing electrode and then to the image surface of the photosensitive element 1, and is collected in the tank 60 and recirculated from drain 61 to the nozzle 53. After the image is developed, sheet 1 is withdrawn from tank 60 by rolls 51 and 51, which press excess developer from the sheet ll. Development electrode 54 is supported by rod 56 adjustably engaging a fixed support, not shown. The rod 56 may be moved upwardly or downwardly, as indicated by arrow 57 to increase or decrease the spacing between electrode 54 and the image surface of sheet ll, thereby controlling background fog and edge effect, as previously described. Alternatively or additionally, rod 56 may be pivoted to vary the electrode spacing at the beginning and end of the development process, for the effects suggested with respect to the FIGS. 6, 7 and 11 embodiments.

Several embodiments of the invention have been described for controlling the spacing between an electrostatic latent image and a development electrode, for obtaining varying edge and fog effects. Other embodiments, and modifications and variations of those described will be apparent to those skilled in the art. Therefore, such embodiments, modifications and variations as are embraced by the spirit and scope of apsaid carrier surface.

pended claims are contemplated as being within the purview of the present invention.

What is claimed is:

1. An apparatus for developing a latent electrostatic image on the surface of an image carrier with liquid developer having toner particles dispersed in an electrically insulating liquid vehicle, comprising means for transporting said carrier through the developer along a determined path, a plurality of development electrodes positioned adjacent said path and having a surface in juxtaposed relationship to said carrier surface during the traverse of said carrier along said path, and means for selectively adjustably varying the position of said electrodes along a line transverse to said path to obtain a desired spacing between said electrode surfaces and said carrier surface during the traverse of said carrier along said path, said electrode adjustment means varying the position of different electrodes differently.

2. An apparatus as set forth in claim 1, wherein said development electrodes are rolls.

3. An apparatus as set forth in claim 1, wherein said electrode adjustment means comprises a support for said electrodes, and pivotable lever means cooperating with said support to alter selectively the position thereof, said lever means varying the position of different electrodes differently.

4. An apparatus as set forth in claim 1, wherein said development electrodes are rolls having a plurality of annular recesses of different depths adjacent each end of said rolls, band means positioned in one of said recesses at each end of said rolls, and projecting therefrom, and means for shifting said band means between said recesses at their respective ends of said rolls, said band means being adapted to engage the side edges of said carrier surface during its traverse of said path and thereby controlling the spacing of the roll surfaces from 

1. An apparatus for dEveloping a latent electrostatic image on the surface of an image carrier with liquid developer having toner particles dispersed in an electrically insulating liquid vehicle, comprising means for transporting said carrier through the developer along a determined path, a plurality of development electrodes positioned adjacent said path and having a surface in juxtaposed relationship to said carrier surface during the traverse of said carrier along said path, and means for selectively adjustably varying the position of said electrodes along a line transverse to said path to obtain a desired spacing between said electrode surfaces and said carrier surface during the traverse of said carrier along said path, said electrode adjustment means varying the position of different electrodes differently.
 2. An apparatus as set forth in claim 1, wherein said development electrodes are rolls.
 3. An apparatus as set forth in claim 1, wherein said electrode adjustment means comprises a support for said electrodes, and pivotable lever means cooperating with said support to alter selectively the position thereof, said lever means varying the position of different electrodes differently.
 4. An apparatus as set forth in claim 1, wherein said development electrodes are rolls having a plurality of annular recesses of different depths adjacent each end of said rolls, band means positioned in one of said recesses at each end of said rolls, and projecting therefrom, and means for shifting said band means between said recesses at their respective ends of said rolls, said band means being adapted to engage the side edges of said carrier surface during its traverse of said path and thereby controlling the spacing of the roll surfaces from said carrier surface. 